Believe it or not although Blender was free to download and use it IS NOT YOURS. It was made, developed, and coded by a Blender Team. In fact, because it is free the license for using Blender is more limited, in that attribution is essentially your payment for Blender services. Those that use Blender do not usually actually read the Terms and Service, so here is an overview on use of images:

There are a few kinds of way you can use images in Blender, and each one requires a specific type of Copyright.

A screenshot is where you basically use the PrintScrn button on your keyboard or the little "Scene" button in Blender to take an immediate picture of what is on your screen now. These often look like this:

Note: These images were taken from other tutorials and are not necessarily mine. They are used in example and attribution is given to their respective authors.

Most things that Blender adds to the inside of the 3D View, such as the grid, are probably ineligible for copyright. But most screenshots of Blender contain things like the tabs and buttons, etc. (even the window itself with nothing in it) which were not made by you and therefore attribution must be given to Blender 3D, along with the license for any content that was made by you.